Portico Next Generation Approval Study

Key Inclusion Criteria: 1. Subjects must have a Society of Thoracic Surgeons (STS) score of ≥7% OR documented heart team agreement of high or extreme risk for surgical aortic valve replacement due to frailty or co-morbidities not captured by the STS score. 2. Subject has symptomatic aortic stenosis as demonstrated by NYHA (New York Heart Association) Functional Classification of II, III, or IV. 3. Subject has senile degenerative aortic valve stenosis with echo-derived criteria, defined as: aortic valve area (AVA) of ≤ 1.0 cm2 (or indexed EOA (effective orifice area) ≤ 0.6 cm2/m2) AND mean gradient ≥40 mmHg or peak jet velocity ≥ 4.0 m/s or doppler velocity index (DVI) ≤0.25. (Qualifying AVA baseline measurement must be within 90 days prior to informed consent). 4. Aortic annulus diameter of 19-30mm and ascending aorta diameter for the specified valve size listed in the IFU (Instructions For Use), as measured by CT (Computed Tomography) Key Exclusion Criteria: 1. Evidence of an acute myocardial infarction (defined as: ST Segment Elevation as evidenced on 12 Lead ECG) within 30 days prior to index procedure. 2. Any percutaneous coronary or peripheral interventional procedure performed within 30 days prior to index procedure. 3. Blood dyscrasias as defined: leukopenia (WBC\<3000 mm3), acute anemia (Hb \< 9 g/dL), thrombocytopenia (platelet count \<50,000 cells/mm³). History of bleeding diathesis or coagulopathy 4. Active peptic ulcer or upper GI bleeding within 3 months prior to index procedure that would preclude anticoagulation 5. Recent (within 6 months prior to index procedure date) cerebrovascular accident (CVA) or a transient ischemic attack (TIA). 6. Mixed aortic valve disease (aortic stenosis and aortic regurgitation with predominant aortic regurgitation 3-4+). 7. Pre-existing prosthetic heart valve or other implant in any valve position, prosthetic ring, severe circumferential mitral annular calcification (MAC) which is continuous with calcium in the left ventricular outflow tract (LVOT), severe (greater than or equal to 3+) mitral insufficiency, or severe mitral stenosis with pulmonary compromise. 8. Minimum access vessel diameter of \<5.0mm for small FlexNav™ Delivery System and \<5.5 mm for large FlexNav™ Delivery System